It is well known to add high molecular weight polymeric retention aid to the cellulosic suspension during the paper-making process. Usually the retention aid is added after the last point of high shear, generally immediately prior to drainage. It is also known to include particulate inorganic material such as bentonite, for instance it may be added to thick stock to reduce pitch problems.
There have been a few instances where it has been proposed to use a substantially non-ionic retention aid, but more usually the retention aid is ionic, most usually cationic.
U.S. Pat. No. 3,052,595 discloses a method of making paper comprising the addition of filler, bentonite and non-ionic acrylamide to the cellulosic suspension. It is disclosed that the polymer can be added to the suspension either before or after the addition of fillers, but the preferred process involves adding bentonite to a cellulosic suspension containing filler, and then adding the polymer. The disclosure is concerned with conventional suspensions and the production of filled, good quality, paper, and the discovery that the inclusion of bentonite with the filler enhances the activity of a non-ionic polymeric retention aid.
U.S. Pat. No. 4,305,781 discloses a method of making paper from a pulp having a high cationic demand by adding bentonite to the stock followed by a substantially non-ionic polyacrylamide as a retention aid. Bentonite is added so as to modify the suspension to render it amenable to treatment by the substantially non-ionic retention aid. In U.S. Pat. No. 4,749,444, a low molecular weight cationic polymer is added after the bentonite and before the non-ionic retention aid, so as to modify the formation properties of the paper.
In U.S. Pat. No. 4,643,801, cationic starch is mixed into the suspension and thereafter an electro-neutralising amount of anionic polymer and dispersed silica are added, generally as a mixture, but it is also mentioned that the anionic polymer may be added followed by the dispersed silica.
In U.S. Pat. No. 4,795,531 low molecular weight cationic polymer is added to the cellulosic suspension to neutralise the charge in the suspension, and subsequently high molecular weight polymer and colloidal silica are added in either order. The high molecular weight polymer can be anionic or cationic.
Despite some usage of non-ionic or anionic retention aids, as indicated above, it is more common to use cationic retention aids. The amount of cationic retention aid that is required generally increases with increasing anionic charge in the suspension.
The cationic polymer that is to be used as a retention aid is normally added after the last point of high shear, but in U.S. Pat. Nos. 4,753,710 and 4,913,775 we describe processes in which a cationic polymer is added, the suspension is then subjected to shearing, and bentonite is then added prior to drainage. It is explained that microflocs are formed by the shearing and that the amount of cationic polymer should be sufficient to render parts at least of the surfaces of the microflocs sufficiently cationically charged, but it is acknowledged that the Zeta potential of the stock prior to addition of the bentonite can be either cationic or anionic. It is stated to be essential to use a cationic polymer rather than a non-ionic or anionic polymer. It is stated that the flocs carry sufficient cationic charge to interact with the bentonite.
These processes have been commercialised very successfully under the trade name "Hydrocol" and they are effective for a wide range of cellulosic suspensions. It is explained in U.S. Pat. No. 4,753,710 that the retention aid should be cationic and is a fact that other retention aids are generally unsatisfactory in that process.
It is alleged in U.S. Pat. No. 5,234,548 (not published until after the priority date of this application) that good results are obtained when the retention aid is an anionic or nonionic polymer but the only detailed proposal for when this might apply is when the suspension is initially dosed with a cationic donor such as alum or a low molecular weight cationic polymer.
The reality is that conventional suspensions can advantageously be dosed with low molecular weight cationic polymer and still be suitably treated with cationic high molecular polymer in the process of U.S. Pat. No. 4,753,710. However the use of cationic retention aid followed by bentonite, as in the Hydrocol process, does prove less satisfactory with some suspensions, and in particular with those have a substantial amount of electrolyte in them, which may arise from the presence of anionic trash, recycling or added materials. Thus, for instance, the processes have been less successful for the treatment of mechanical pulps such as groundwood and thermo-mechanical pulps; dirty pulps such as crude pulps traditionally used for newsprint manufacture; and recycled pulps such as de-inked waste; and for the treatment of suspensions in closed mills wherein the whitewater is repeatedly recycled with the introduction of only low amounts of fresh water into the process. The anionic trash arises from impurities in the mechanical pulps. The high electrolyte content may alternatively arise from, for instance, the use of filler that is liable to render the white water alkaline due to partial dissolution of the filler, for instance calcium sulphate or calcium carbonate.
Suspensions which contain high electrolyte levels are generally anionic and conventional thinking would suggest that increased amounts of cationic polymer should be added to reduce or eliminate the anionic nature of the suspension.
Processes involving the application of cationic starch and colloidal silicic acid or other modified silicas are described in U.S. Pat. No. 4,388,150 and have been commercialised under the trade name "Composil". In general, these processes are applicable to a narrower range of suspensions than the "Hydrocol" process.
It would be desirable to be able to devise a dewatering process for the manufacture of paper that can have, in particular, good dewatering performance (retention, drainage and/or drying) and formation properties as good as the "Hydrocol" process, using a pulp having a high electrolyte content rather than a conventional pulp that typically works with the "Hydrocol" process using cationic retention aid. In particular, it would be desirable to be able to obtain benefits similar to those of the "Hydrocol" process in a cost-effective manner when treating a cellulosic suspension that contains significant amounts of anionic trash.